U.S. patent application number 10/235437 was filed with the patent office on 2004-03-11 for protocol for addressing groups of rfid tags.
This patent application is currently assigned to Honeywell International Inc.. Invention is credited to Becker, Robert C., Kazi, Sabera, Meyers, David W..
Application Number | 20040046642 10/235437 |
Document ID | / |
Family ID | 31977559 |
Filed Date | 2004-03-11 |
United States Patent
Application |
20040046642 |
Kind Code |
A1 |
Becker, Robert C. ; et
al. |
March 11, 2004 |
Protocol for addressing groups of RFID tags
Abstract
A tagging system includes a tag reader and a plurality of RFID
tags. The tag reader addresses a group of RFID tags, and the group
of RFID tags is a subset of the plurality of RFID tags. Each of the
RFID tags in the plurality of RFID tags has a unique identifier.
The tag reader transmits a group address having first and second
sets of data elements. The data elements in the first set of data
elements are substantially equal in value to corresponding ones of
the data elements in the identifiers of the group of RFID tags. The
data elements in the second set of data elements represent any
value for corresponding ones of the data elements in the
identifiers of the group of RFID tags. Only the RFID tags having
corresponding data elements in their identifiers that match the
first set of data elements in the group address respond.
Inventors: |
Becker, Robert C.; (Eden
Prairie, MN) ; Meyers, David W.; (Brooklyn Park,
MN) ; Kazi, Sabera; (Minneapolis, MN) |
Correspondence
Address: |
HONEYWELL INTERNATIONAL INC.
101 COLUMBIA ROAD
P O BOX 2245
MORRISTOWN
NJ
07962-2245
US
|
Assignee: |
Honeywell International
Inc.
|
Family ID: |
31977559 |
Appl. No.: |
10/235437 |
Filed: |
September 5, 2002 |
Current U.S.
Class: |
340/10.32 ;
340/9.11 |
Current CPC
Class: |
G06K 7/10039 20130101;
G01V 15/00 20130101; G06K 7/0008 20130101 |
Class at
Publication: |
340/010.32 ;
340/825.52 |
International
Class: |
H04Q 005/22; H04Q
001/00 |
Claims
We claim:
1. A method of addressing a group of RFID tags, wherein the group
of RFID tags comprises a subset of a plurality of RFID tags capable
of being addressed by a tag reader, wherein each of the RFID tags
in the plurality of RFID tags has a unique identifier, and wherein
the method comprises: inserting a group address into a message,
wherein the group address comprises a first set of data elements
substantially equal in value to corresponding ones of the data
elements in the identifiers of the group of RFID tags, and wherein
the group address comprises a second set of data elements
representing any value for corresponding ones of the data elements
in the identifiers of the group of RFID tags; and, transmitting the
message to the plurality of RFID tags.
2. The method of claim 1 wherein each of the identifiers comprises
K data elements, wherein the first set of data elements comprises L
data elements, wherein the second set of data elements comprises M
data elements, and wherein L+M=K.
3. The method of claim 1 further comprising: receiving reply
messages; increasing a size of the group address if no reply
messages collided; and, decreasing the size of the group address if
at least some of the reply messages collided.
4. The method of claim 1 wherein the first set of data elements
comprises L data elements, wherein the second set of data elements
comprises M data elements, and wherein the inserting of a group
address into a message comprises inversely adjusting L and M
depending upon replies received in response to the message.
5. The method of claim 4 wherein each of the identifiers comprises
K data elements, and wherein L+M=K.
6. The method of claim 1 further comprising: receiving reply
messages; estimating a number of the reply messages that collide;
and, determining a size for the group address in accordance with
the estimated number.
7. The method of claim 6 wherein the first set of data elements
comprises L data elements, wherein the second set of data elements
comprises M data elements, and wherein the inserting of a group
address into a message comprises inversely adjusting L and M in
accordance with the determined size.
8. The method of claim 7 wherein each of the identifiers comprises
K data elements, and wherein L+M=K.
9. The method of claim 6 wherein the determining of a size for a
group address comprises increasing the group size when the number
of colliding reply messages decreases and decreasing the group size
when the number of colliding reply messages increases.
10. The method of claim 1 wherein the message comprises a first
message, wherein the method further comprises transmitting a second
message to the plurality of RFID tags, and wherein the second
message contains an address of a specific one of the plurality of
RFID tags.
11. The method of claim 10 wherein each of the identifiers
comprises K data elements, wherein the address of a specific one of
the plurality of RFID tags comprises L data elements, and wherein
L=K.
12. The method of claim 1 further comprising receiving reply
messages and storing data contained in the reply messages.
13. The method of claim 1 further comprising: receiving data in
time slots; increasing a size of the group address if at least one
of the time slots contains no data; and, decreasing the size of the
group address if at least some of the time slots contain collided
data.
14. A method of processing a message received from a tag reader,
wherein the message contains a group address comprising K data
elements, wherein the group address comprises first and second sets
of data elements, wherein the first set of data elements comprises
L data elements, wherein the second set of data elements comprises
M data elements, wherein L+M=K, wherein L.noteq.0, wherein
M.noteq.0, wherein K.noteq.0, and wherein the method comprises:
comparing the group address to an identifier, wherein the
identifier comprises K data elements; and, determining a match even
if only the first set of data elements favorably compares to
corresponding data elements in the identifier.
15. The method of claim 14 wherein the comparing of the group
address to an identifier comprises comparing only the first set of
data elements in the group address to corresponding data elements
in the identifier.
16. The method of claim 14 further comprising transmitting a reply
message only if the match is determined.
17. The method of claim 14 wherein the message comprises a first
message, wherein the group address comprises a first address,
wherein the match comprises a first match, and wherein the method
further comprises: receiving a second message, wherein the second
message contains a second address, wherein the second address
contains N data elements, and wherein N=K; comparing the second
address to the identifier; and, determining a second match only if
all of the N data elements of the second address favorably compare
to corresponding data elements in the identifier.
18. The method of claim 17 further comprising transmitting a first
reply message only if the first match is determined and
transmitting a second reply message only if the second match is
determined.
19. A method of processing a message from a tag reader, wherein the
message contains a group address, wherein the group address
addresses a group of RFID tags, wherein the group of RFID tags
comprises a subset of a plurality of RFID tags capable of being
addressed by the tag reader, wherein each of the RFID tags in the
plurality of RFID tags has a unique identifier, wherein the group
address comprises a first set of data elements substantially equal
in value to corresponding ones of the data elements in the
identifiers of the group of RFID tags, wherein the group address
comprises a second set of data elements representing any value for
corresponding ones of the data elements in the identifiers of the
group of RFID tags, and wherein the method comprises: receiving the
message; comparing the group address contained in the received
message to one of the identifiers; and, determining a match if each
of the first set of data elements favorably compares to a
corresponding data element in the one identifier.
20. The method of claim 19 further comprising transmitting a reply
message only if the match is determined.
21. The method of claim 19 wherein each of the identifiers
comprises K data elements, wherein the first set of data elements
comprises L data elements, wherein the second set of data elements
comprises M data elements, and wherein L+M=K.
22. The method of claim 19 wherein the message comprises a first
message, wherein the group address comprises a first address,
wherein the match comprises a first match, and wherein the method
further comprises: receiving a second message, wherein the second
message contains a second address of a specific one of the
plurality of RFID tags; comparing the second address to one of the
identifiers; and, determining a second match only if all data
elements of the second address favorably compares to corresponding
data elements in the one identifier.
23. The method of claim 22 further comprising transmitting a first
reply message only if the first match is determined and
transmitting a second reply message only if the second match is
determined.
24. The method of claim 22 wherein each of the identifiers
comprises K data elements, wherein the second address comprises N
data elements, and wherein N=K.
25. The method of claim 22 wherein each of the identifiers
comprises K data elements, wherein the first set of data elements
comprises L data elements, wherein the second set of data elements
comprises M data elements, and wherein L+M=K.
26. The method of claim 25 wherein the second address comprises N
data elements, and wherein N=K.
27. The method of claim 19 wherein the received message comprises a
first received message, wherein the group address comprises a first
group address, wherein the method further comprises receiving
second and third messages containing second and third respective
group addresses, wherein the second group address covers a larger
group of RFID tags than the group of RFID tags covered by the first
group address as a result of fewer collisions in a previous reply
message, and wherein the third group address covers a smaller group
of RFID tags than the group of RFID tags covered by the first group
address as a result of more collisions in a previous reply message
Description
TECHNICAL FIELD OF THE INVENTION
[0001] The present invention relates to the addressing of RFID tags
by a tag reader.
BACKGROUND OF THE INVENTION
[0002] Various labels have been attached to articles so that the
articles can be distinguished one from the other. For example, bar
code labels are attached to articles of grocery and are scanned at
a check-out counter in order to automatically identify the articles
and to register the price of the articles as they are
purchased.
[0003] Bar code labels have also been used in inventory control and
monitoring. Accordingly, these bar codes may be scanned in order to
track articles as they move into, through, and out of a storage
area. It is also known to read the bar codes attached to articles
in order to access various computer records regarding the
articles.
[0004] Bar code labels, however, have several drawbacks. For
example, computer stored records that are accessed when a bar code
is read do not move with the corresponding article. Therefore, if
the article to which the bar code label is attached is remote from
the computer, the records concerning that article cannot be
immediately accessed if necessary.
[0005] Moreover, bar code labels cannot be read remotely. Thus, if
it is desired to take an inventory of articles currently in the
storage area, personnel must physically scan each label on each
article one at a time in order to determine which articles are
presently in the storage area. Such scanning requires the physical
presence of the personnel at the location of the articles and is
extremely time consuming. Additionally, because bar code labels
cannot be read remotely, they cannot be used as security devices
that can be detected if the articles to which they are attached are
improperly removed from a secured area.
[0006] Instead of bar coded labels, it is known to attach radio
frequency identification (RFID) tags to the articles to be
monitored. As in the case of bar code labels, the RFID tags contain
unique identifiers so that the articles to which they are attached
can be distinguished from one another. However, unlike bar code
labels, reading RFID tags does not require the physical presence of
personnel because the RFID tags can instead be read remotely.
[0007] Accordingly, inventory can be taken more quickly because
personnel are not required to walk around a storage area or other
area in order to read the RFID tags. Moreover, because RFID tags
can be read remotely, they can be used as security devices. Thus,
if someone attempts to surreptitiously remove an article to which
an RFID tag is attached from a secured area, a remote reader can
sense the RFID tag and provide an appropriate alarm. Furthermore,
it is also possible to provide an RFID tag with memory. Therefore,
any records concerning the article to which the RFID tag is
attached can be maintained on the RFID tag rather than in a
remotely located computer.
[0008] When reading the IDs and/or other information from RFID
tags, a tag reader can address the RFID tags individually,
requiring the addressed tag to transmit back reply messages
containing the requested information. Alternatively, the tag reader
can address all of the tags simultaneously. In this case, all tags
will transmit back reply messages containing the desired
information. When multiple tags transmit reply messages, there may
be a chance of collisions between the reply messages resulting in a
loss of information.
[0009] A variety of methods have been employed to avoid collisions
between the reply messages being transmitted back by the RFID tags
to the tag reader. For example, RFID tags have been arranged to
randomly select the time slots within which they transmit their
reply messages.
[0010] It is also useful to interrogate some but not all of the
available RFID tags. For example, articles of different types may
be stored in one or more storage areas covered by the same tag
reader. The articles can be divided into a multiplicity of groups
according to a variety of criteria. An example of such a grouping
could be by the item type, and each group can be assigned a set of
addresses capable of being defined by a unique group address. Thus,
when it is desired to take separate inventories of each article
type, each group may be individually addressed by its group address
and required to respond with appropriate information. The present
invention is directed to a technique to perform group addressing so
that some but not all RFID tags are required to respond.
SUMMARY OF THE INVENTION
[0011] In accordance with one aspect of the present invention, a
method is provided to address a group of RFID tags. The group of
RFID tags comprises a subset of a plurality of RFID tags capable of
being addressed by a tag reader, and each of the RFID tags in the
plurality of RFID tags has a unique identifier. The method
comprises the following: inserting a group address into a message,
wherein the group address comprises a first set of data elements
substantially equal in value to corresponding ones of the data
elements in the identifiers of the group of RFID tags, and wherein
the group address comprises a second set of data elements
representing any value for corresponding ones of the data elements
in the identifiers of the group of RFID tags; and, transmitting the
message to the plurality of RFID tags.
[0012] In accordance with another aspect of the present invention,
a method is provided to process a message received from a tag
reader. The message contains a group address comprising K data
elements, and the group address comprises first and second sets of
data elements. The first set of data elements comprises L data
elements, the second set of data elements comprises M data
elements, and L+M=K, where L.noteq.0, M.noteq.0, and K.noteq.0. The
method comprises the following: comparing the group address to an
identifier, wherein the identifier comprises K data elements; and,
determining a match even if only the first set of data elements
favorably compares to corresponding data elements in the
identifier.
[0013] In accordance with still another aspect of the present
invention, a method is provided to process a message from a tag
reader. The message contains a group address, and the group address
addresses a group of RFID tags. The group of RFID tags comprises a
subset of a plurality of RFID tags capable of being addressed by
the tag reader, and each of the RFID tags in the plurality of RFID
tags has a unique identifier. The group address comprises a first
set of data elements that are substantially equal in value to
corresponding ones of the data elements in the identifiers of the
group of RFID tags. The group address also comprises a second set
of data elements that represent any value for corresponding ones of
the data elements in the identifiers of the group of RFID tags. The
method comprises the following: receiving the message; comparing
the group address contained in the received message to one of the
identifiers; and, determining a match if each of the first set of
data elements favorably compares to a corresponding data element in
the one identifier.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] These and other features and advantages will become more
apparent from a detailed consideration of the invention when taken
in conjunction with the drawings in which:
[0015] FIG. 1 illustrates a tagging system in accordance with one
embodiment of the present invention;
[0016] FIG. 2 illustrates additional detail a representative one of
the tags of the tagging system of FIG. 1;
[0017] FIG. 3 illustrates additional detail of a tag reader that
can be used with the tagging system of FIG. 1;
[0018] FIG. 4 illustrates a message format useful in supporting
communications between the tag reader and the tags of the tagging
system shown in FIG. 1;
[0019] FIG. 5 illustrates an exemplary composition of a frame of
the message format shown in FIG. 4;
[0020] FIG. 6 illustrates an exemplary composition of the header of
the frame shown in FIG. 5;
[0021] FIG. 7 illustrates an exemplary composition of a time slot
of the frame shown in FIG. 5;
[0022] FIG. 8 illustrates an exemplary composition of the header of
the time slot shown in FIG. 7;
[0023] FIGS. 9 and 10 are flow charts showing an exemplary
operation of the tag reader of FIGS. 1 and 3; and,
[0024] FIG. 11 is a flow chart showing an exemplary operation of
the tags illustrated in FIGS. 1 and 2.
DETAILED DESCRIPTION
[0025] As illustrated in FIG. 1, a tagging system 10 includes a tag
reader 12, and a plurality of RFID tags 14.sub.1 through 14.sub.n,
where n can be any desired number greater than one. The tag reader
12, for example, may be a long range reader capable of reading data
from the RFID tags 14.sub.1 through 14.sub.n over a substantial
distance, such as several hundred feet or more, although the tag
reader 12 may have an expected range of anywhere from less than 5
feet up to approximately 500 feet.
[0026] The tag reader 12 includes an antenna 16, and each of the
RFID tags 14.sub.1 through 14.sub.n similarly includes an antenna
18. The antennas 16 and 18 establish an RF link between the tag
reader 12 and each of the RFID tags 14.sub.1 through 14.sub.n so
that the tag reader 12 can remotely read the identifiers and/or
other information stored in memories of the RFID tags 14.sub.1
through 14.sub.n.
[0027] Each of the RFID tags 14.sub.1 through 14.sub.n has
associated therewith an identifier that uniquely identifies it. The
identifier that uniquely identifies each of the RFID tags 14.sub.1
through 14.sub.n comprises a plurality of data elements, and a data
element can be a bit, a group of bits, a symbol, a group of
symbols, etc. Accordingly, each one of the RFID tags 14.sub.1
through 14.sub.n can be distinguished from all other ones of the
RFID tags 14.sub.1 through 14.sub.n by its unique identifier.
[0028] If desired, a second reader (not shown), such as a short
range reader, may be provided to read data from the RFID tags
14.sub.1 through 14.sub.n from only a short distance, such as less
than two feet, although the short range reader may be expected, in
typical usage, to have a range of between six inches and eighteen
inches. A secure link (also not shown) between the second reader
and the RFID tags 14.sub.1 through 14.sub.n permits the second
reader to read information from the RFID tags 14.sub.1 through
14.sub.n in a more secure manner. For example, it may not be
desirable for the tag reader 12 to read certain information stored
in the RFID tags 14.sub.1 through 14.sub.n because long range RF
communications can be intercepted by a strategically placed
surreptitious reader similar to the tag reader 12. Accordingly, the
secure link provided by the second reader increases the difficulty
in illicitly acquiring the more sensitive information that may be
stored on the RFID tags 14.sub.1 through 14.sub.n.
[0029] This secure link may a hard wire link between the second
reader and the RFID tags 14.sub.1 through 14.sub.n. Alternatively,
the secure link may be a limited range magnetic link such as those
provided by contact-free smart cards. As a still further
alternative, the secure link may be a limited range RF link. Other
alternatives will occur to those skilled in the art. Accordingly,
the more sensitive information stored on the RFID tags 14.sub.1
through 14.sub.n can be read over the secure link between the
second reader and the RFID tags 14.sub.1 through 14.sub.n.
[0030] One advantage of using a non-hardwired secure link is that
then the RFID tags 14.sub.1 through 14.sub.n can be more readily
used as security devices. Accordingly, when an attempt is made to
remove an article to which one of the RFID tags 14.sub.1 through
14.sub.n is attached, the second reader located at a portal of a
secured area, or otherwise, can elicit and detect a possibly
specific signal from the RFID tag indicating that an attempt is
being made to remove the article from the secured area.
[0031] Each of the RFID tags 14.sub.1 through 14.sub.n may have the
same architecture. Accordingly, only one of the RFID tags 14.sub.1
through 14.sub.n, i.e., the RFID tag 141, is shown in additional
detail in FIG. 2. The RFID tag 14.sub.1 includes a transceiver 30
comprising a frequency agile (frequency hopping) RF transmitter 32
and a direct sequence spread spectrum RF receiver 34. The frequency
agile RF transmitter 32 and the direct sequence spread spectrum RF
receiver 34 are coupled between the antenna 18 and a microprocessor
36. Accordingly, the frequency agile RF transmitter 32 of the RFID
tag 14.sub.1 implements frequency hopping in transmitting
information to the tag reader 12, and the direct sequence spread
spectrum RF receiver 34 of the RFID tag 14.sub.1 implements direct
sequence spread spectrum synchronization and decoding in receiving
communications from the tag reader 12.
[0032] The RFID tag 14.sub.1 may also include a second transceiver
(not shown) between the microprocessor 36 and the second reader
discussed above. Accordingly, the RFID tag 14.sub.1 can transmit
and/or receive communications to and/or from this second reader. In
the case where the secure link between the second reader and the
RFID tag 14.sub.1 is a hardwire link, the second transceiver may
simply be a plug that is connectible to a corresponding plug of the
second reader. In the case where the secure link is an RF link, the
second transceiver may be an RF transceiver of any known type,
provided that this RF transceiver preferably has a shorter range
than the frequency agile RF transmitter 32 and the direct sequence
spread spectrum RF receiver 34. In the case where the secure link
is a magnetic link, the second transceiver may simply be a magnetic
emitter (and/or sensor) capable of magnetically interfacing with
the second reader.
[0033] The RFID tag 14.sub.1 further comprises a memory 38 coupled
to the microprocessor 36. The memory 38 stores the unique
identifier of the RFID tag 14.sub.1 that can be read by the tag
reader 12 through the antennas 16 and 18, the frequency agile RF
transmitter 32, the direct sequence spread spectrum RF receiver 34,
and the microprocessor 36. The memory 38 may also store other
information supplied to it by the second reader through the secure
link, the second transceiver, and the microprocessor 36. The memory
38 can additionally store information supplied by the tag reader
12.
[0034] In addition to the unique identifier, the information stored
in the memory 38 can include, for example, the inventory history of
the article to which the RFID tag 14.sub.1 is attached.
Accordingly, the date that the article entered inventory, the date
that the article left inventory, the length of time that the
article has been in inventory, any movement within inventory, and
similar information may be stored in the memory 38.
[0035] The information stored in the memory 38 may also include
shipping manifests that indicate when and to whom the article is to
be shipped. Moreover, in the case where individual articles with
differing destinations are shipped in the same container, an RFID
tag attached to the container, hereafter called a container tag,
can be attached to the container. This container tag may be
arranged to store the identity and destination of each article in
the container. As articles are removed from the container, the
information stored in the container tag can be updated to indicate
which articles have been removed, the location at which the
articles were removed, and the identity of the personnel who
removed the articles.
[0036] The information stored in the memory 38 may further include
maintenance, repair, and date of service records showing the
maintenance and/or repair history of the corresponding article.
[0037] Other information related to the article may likewise be
stored in the memory 38. For example, the integrity of the
information stored in the memory 38 can be assured by keeping a
record of the modifications to the stored information and of the
identity of the personnel making the modifications. As another
example, records related to the production of the article may be
stored in the memory of the tag.
[0038] Accordingly, any information about the article may be stored
with the article instead of in a remote computer system or on
paper.
[0039] Because the records are carried by the RFID tag 14.sub.1
attached to a corresponding article, the RFID tag 14.sub.1
eliminates the need to maintain paper or computer records of the
life history of an article, the RFID tag 14.sub.1 eliminates the
problem of lost or misplaced records, and the RFID tag 14.sub.1
improves operational efficiency by eliminating the requirement to
retrieve records prior to accessing and/or operating on the
article.
[0040] The RFID tag 14.sub.1 may additionally include a battery
(not shown) that is coupled so that it supplies power to the
frequency agile RF transmitter 32, to the direct sequence spread
spectrum RF receiver 34, to the microprocessor 36, and to the
memory 38 (if necessary).
[0041] Moreover, a plurality of sensors (not shown) may be coupled
to the microprocessor 36. These sensors may include, for example, a
temperature sensor, a humidity sensor, and other sensors such as a
pressure sensor, a proximity sensor, an electromagnetic sensor, an
optical sensor, a mechanical sensor, a chemical sensor, and/or the
like. The microprocessor 36 stores the information from the sensors
in the memory 38, and this information may be read from the memory
38 by the tag reader 12 (and/or by the second reader).
[0042] The microprocessor 36 may be further arranged to sense the
voltage level of the battery. Accordingly, the microprocessor 36
stores this voltage level in the memory 38, and this stored voltage
level may be read from the memory 38 by the tag reader 12 (and/or
by the second reader). Thus, if the voltage level of the battery as
read by the tag reader 12 (and/or by the second reader) indicates
that the battery needs charging or replacement, suitable remedial
action may be taken.
[0043] Because of the frequency agile RF transmitter 32 and the
direct sequence spread spectrum RF receiver 34, the RFID tag
14.sub.1 is capable of relatively long range activation while
providing a low power method for command-response activation by the
tag reader 12. This long range activation allows the RFID tag
14.sub.1 to be placed at distances remote from the tag reader 12
for purposes of interrogating the RFID tag 14.sub.1 for its unique
identifier and possibly other information.
[0044] The frequency agile RF transmitter 32 and the direct
sequence spread spectrum RF receiver 34 allow the tagging system 10
to operate in the FCC defined Industrial Scientific and Medical
(ISM) bands at maximum legal power. Both frequency hopping as used
by the frequency agile RF transmitter 32 and direct sequence spread
spectrum communications as used by the direct sequence spread
spectrum RF receiver 34 circumvent jamming by narrow-band signals
using different methods of spreading the signal over a large
bandwidth. The direct sequence spread spectrum RF receiver 34 can
receive signals from the tag reader 12 within milliseconds of
activation. By contrast, a frequency agile receiver must search a
long frequency hopping sequence in order to receive signals from
the tag reader 12. The time required to make this search is
typically longer than the time required to detect a direct spread
spectrum sequence because the direct spread spectrum signal is
either on a fixed frequency or on one of only a few
frequencies.
[0045] An embodiment of the tag reader 12 is shown in additional
detail in FIG. 3. The tag reader 12 includes a direct sequence
spread spectrum RF transmitter 50 and a frequency agile RF receiver
52 coupled between the antenna 16 and a microprocessor 54. The
frequency agile RF receiver 52 of the tag reader 12 implements
frequency hopping in receiving information from the frequency agile
RF transmitter 32 of the RFID tags 14.sub.1 through 14.sub.n.
Moreover, the direct sequence spread spectrum transmitter 50 of the
tag reader 12 implements direct sequence spread spectrum
transmission in transmitting communications to the direct sequence
spread spectrum RF receiver 34 of the RFID tags 14.sub.1 through
14.sub.n.
[0046] The tag reader 12 further comprises a memory 56 coupled to
the microprocessor 54. The memory 56 stores the information that
the tag reader 12 receives from the RFID tags 14.sub.1 through
14.sub.n. The memory 56 also stores the software that supports a
communication protocol as described herein.
[0047] This communication protocol governs the message format that
is used between the tag reader 12 and the RFID tags 14.sub.1
through 14.sub.n. According to one embodiment of the communication
protocol that governs the message format that is used between the
tag reader 12 and the RFID tags 14.sub.1 through 14.sub.n, a
message may be comprised of a plurality of frames as shown in FIG.
4. Each frame is preferably no longer than the length of time that
the frequency agile RF transmitter 32 is allowed to dwell at any
given frequency.
[0048] Each of the frames shown in FIG. 4 has the construction
shown in FIG. 5. Accordingly, each frame has a frame header and a
number of time slots TS0-TSN. The frame header contains information
about the tag reader 12 that is reading the RFID tags 14.sub.1
through 14.sub.n. As shown in FIG. 6, the header contains (i) the
state of the tag reader 12, (ii) the hop sequence currently being
used by the tag reader 12 to receive messages from the RFID tags
14.sub.1 through 14.sub.n, and (iii) the current position (i.e.,
frequency) of the tag reader 12 in this hop sequence. The frame
header can also contain such other information as may be useful in
the tagging system 10. For example, the frame header may also
contain the number (N+1) of the time slots in the corresponding
frame, an individual address of a specific one of the RFID tags
14.sub.1 through 14.sub.n, and/or a group address of a subset of
the RFID tags 14.sub.1 through 14.sub.n.
[0049] The tag reader 12 may have several reader states including,
for example, an active communication state and a beacon state. In
the active communication state, the tag reader 12 commands
responses from one or more of the RFID tags 14.sub.1 through
14.sub.n. In the beacon state, the RFID tags 14.sub.1 through
14.sub.n self-initiate the transmission of messages to the tag
reader 12.
[0050] The hop sequence and/or the current position in the hop
sequence as contained in the frame header are/is useful to tags
that have limited signal processing capability. Such tags, for
example, may have no capability themselves to determine the
frequency (i.e., the current position in the hop sequence) onto
which they should transmit their responses.
[0051] Moreover, each time slot may also include a time slot header
and data portion as shown in FIG. 7, and each time slot header, as
shown in FIG. 8, may contain the hop sequence and the current
position in the hop sequence of the tag reader 12. The time slot
header may also contain the relative position, such as a time slot
number (0, 1, . . . , or N), of the corresponding time slot in the
frame. This relative position information may be used by the RFID
tags 14.sub.1 through 14.sub.n to establish a relative timing
interval into which the RFID tags 14.sub.1 through 14.sub.n can
transmit data. By transmitting the hop sequence and the current
position in the hop sequence at the beginning of each time slot,
the RFID tags 14.sub.1 through 14.sub.n are aided in their rapid
acquisition of the current hop sequence and frequency. Because the
RFID tags 14.sub.1 through 14.sub.n can acquire, from the header in
each time slot, sufficient information about the frequency and
timing of the tag reader 12, the RFID tags 14.sub.1 through
14.sub.n may power down until such time that they expect the
complete header information to be transmitted by the tag reader 12.
Therefore, the RFID tags 14.sub.1 through 14.sub.n are able to
substantially reduce the amount of power that they use to determine
the frequency and timing to be used by their frequency agile RF
transmitter 32 in transmitting information in the data portion of
the time slot.
[0052] As indicated above, the tag reader 12 transmits all headers,
whether frame headers or time slot headers. The RFID tags 14.sub.1
through 14.sub.n transmit only in the data portion of the time
slots. Each of the RFID tags 14.sub.1 through 14.sub.n may
implement a non-deterministic method of selecting a time slot for
the transmission of its data. By using a non-deterministic method
of selecting a time slot, the possibility of a plurality of tags
transmitting data into the same time slot is minimized. For
purposes of illustration, such a non-deterministic method of
selecting a time slot could be embodied by a pseudo-random number
generator that pseudo-randomly generates the number of a time slot
into which its corresponding tag transmits its data. This
implementation results in a communications protocol similar to, but
not identical to, the Aloha protocol, a standard communications
protocol.
[0053] Alternatively, the RFID tags 14.sub.1 through 14.sub.n may
transmit in a time slot determined by their identifiers stored in
the memory 38. For example, if there are more tags than there are
time slots in a single frame, the more significant data elements in
the identifiers assigned to the RFID tags 14.sub.1 through 14.sub.n
may be used to designate a frame, and the less significant data
elements in the identifier assigned to the RFID tags 14.sub.1
through 14.sub.n may be used to designate a time slot in the frame
designated by the more significant data elements.
[0054] Accordingly, the RFID tags 14.sub.1 through 14.sub.n of the
tagging system 10 should transmit their information to the tag
reader 12 in a way than minimizes the likelihood of contention. If
more than one of the RFID tags 14.sub.1 through 14.sub.n should
transmit in the same time slot, the tag reader 12 can command the
non-contending ones of the RFID tags 14.sub.1 through 14.sub.n to
be silent and command only the contending tags (or a reduced number
of tags that includes the contending tags) to respond to the next
interrogation from the tag reader 12.
[0055] The tag reader 12 can communicate directly with a specific
tag or a group of specific tags. When the tag reader 12 is
communicating directly with a specific tag or a group of specific
tags, the tag reader 12 may suspend the transmission of time slot
headers. This suspension indicates to all other tags that their
communications are to be suspended. Also, all data may be
transmitted between the tag reader 12 and the RFID tags 14.sub.1
through 14.sub.n in packets having packet numbers so that both the
tag reader 12 and the RFID tags 14.sub.1 through 14.sub.n can
detect missing or duplicate data. Moreover, acknowledgements can be
used to signify a successful transmission between the tag reader 12
and the RFID tags 14.sub.1 through 14.sub.n. A failure to receive
an acknowledgement can cause re-transmission of the information.
Once a transaction between the tag reader 12 and either a specific
one of the RFID tags 14.sub.1 through 14.sub.n or a group of the
RFID tags 14.sub.1 through 14.sub.n is complete, the tag reader 12
resumes transmitting the headers.
[0056] The tag reader 12 can read the RFID tags 14.sub.1 through
14.sub.n by selective activation and/or deactivation of the RFID
tags 14.sub.1 through 14.sub.n by groups. A group of the RFID tags
14.sub.1 through 14.sub.n can be interrogated by requiring only
those of the RFID tags 14.sub.1 through 14.sub.n having identifiers
matching a bit pattern transmitted by the tag reader 12 to
respond.
[0057] For example, the tag reader 12 may transmit a header of a
signal including (i) an indication that the signal is an
interrogation signal, (ii) an indication that the signal contains a
group address, and (iii) the group address. For purposes of this
example, it is assumed that each of the RFID tags 14.sub.1 through
14.sub.n has a twelve bit identifier, and if it is assumed that the
group address is 1110001110xxx, where 0 is a zero bit in a tag
identifier, 1 is a one bit in a tag identifier, and x means a bit
in the tag identifier of any value (frequently referred to as a
"don't care" bit), then all of the RFID tags 14.sub.1 through
14.sub.n having 111000110 for the first nine bits of their
identifiers will respond to the interrogation signal.
[0058] As an other example, if it is assumed that the group address
is 01000xxx1101, where again 0 is a zero bit in a tag identifier, 1
is a one bit in a tag identifier, and x means a bit in the tag
identifier of any value, then all of the RFID tags 14.sub.1 through
14.sub.n having 01000 for the first five bits and 1101 for the last
four bits of their identifiers will respond to the interrogation
signal.
[0059] Although the x's ("don't care" bits) in the above examples
are shown as being grouped together in a group address, they
instead may be scattered throughout the group address such that,
using the assumed twelve bit identifier discussed above, the group
address may be 0x11x010xx11, for example.
[0060] As shown in FIG. 9, when the tag reader 12 is in the
transmit mode (block 100), and if the tag reader 12 determines at a
block 102 that just one of the RFID tags 14.sub.1 through 14.sub.n
is to be addressed, the tag reader 12 at a block 104 inserts the
state of tag reader (the active communication state and a single
tag is being addressed) and the address (identifier) of the
addressed one of the RFID tags 14.sub.1 through 14.sub.n into a
header. The tag reader 12 then transmits the header (block 106) and
sets its mode to the receive mode (block 108).
[0061] However, when the tag reader 12 is in the transmit mode
(block 100), and if the tag reader 12 determines at the block 102
that a group of the RFID tags 14.sub.1 through 14.sub.n is to be
addressed, the tag reader 12 at a block 110 inserts the state of
tag reader (the active communication state and that a group of tags
is being addressed) and the group address (an identifier with
designated data element positions set to x) of the addressed group
of the RFID tags 14.sub.1 through 14.sub.n into a header. The tag
reader 12 then transmits the header (block 106) and sets its mode
to the receive mode (block 108).
[0062] When the tag reader 12 is in the receive mode (block 100),
it waits to receive a reply message from one or more of the RFID
tags 14.sub.1 through 14.sub.n (FIG. 10; block 112). When the tag
reader 12 receives a reply message, it determines whether there
were any collisions between tag replies (block 114). If the tag
reader 12 determines that contention occurred between two or more
tags, the is tag reader 12 estimates the number of collisions that
took place (block 116) and sets the group size for a group address
in accordance with the number of colliding tags (block 118). On the
other hand, if the tag reader 12 detects no collisions at the block
114, and in fact determines that the reply message (such as one or
more time slots) is empty, the tag reader at the block 118 likewise
adjusts the group size accordingly.
[0063] The tag reader 12 at the block 118 sets the group size with
the objective of filling up the time slots of a frame while at the
same time minimizing or eliminating contention between the
addressed RFID tags. Accordingly, when there are empty time slots
in the return frame, the tag reader 12 at the block 118 increases
the group size so that, when the next group address is transmitted,
the time slots will be more efficiently utilized. A larger group
address may be implemented, for example, by increasing the number
of x's in the group address.
[0064] On the other hand, when collisions have occurred in the
received message, the tag reader 12 at the block 118 decreases the
group size so that, when the next group address is transmitted,
collisions in the reply message are reduced or are avoided
altogether. A smaller group address may be implemented, for
example, by decreasing the number of x's in the group address.
[0065] The tag reader 12 can adjust the size of the group address
without estimating the number of collisions at the block 116. For
example, the tag reader 12 may be arranged to eliminate an x from
the group address one at a time until a reply message free of
colliding data is received. However, it is more efficient to
estimate the number of collisions in a reply message so that the
tag reader can more quickly reach a steady state where all time
slots contain data and there are no empty time slots. Accordingly,
the tag reader 12 will converge on the optimum utilization of time
slots more efficiently. In determining the number of collisions,
the tag reader 12 may use any known method. One method of
estimating the number of collisions is for the reader 12 to count
the number of time slots in which a signal is present at or above a
predetermined signal strength at which correct reception is greater
than 99% probable yet the signal in the time slot cannot be
correctly decoded. This implies that the signal has been corrupted
and the most likely source of corruption is a collision between two
or more tag transmissions. For example, if a simple majority of the
time slots are determined to be occupied yet corrupted, the
likelihood of a similar result from repeating the interrogation
unchanged is quite high. In such a case, a reduction in group size
would be in order. A determination of which "don't care" bits in
the group address to remove can be made by examining the
commonality of the messages that were successfully received
vis--vis the group address and selecting one of the "don't care"
address bits that are not common to the addresses received, as
these bits are more likely to be present in more tags and, hence,
result in more transmitting devices and consequently more message
collisions. Other methods may be obvious to those skilled in the
art.
[0066] In the setting of the group size, the tag reader 12 may wait
for the reception of plural time slots.
[0067] Such an arrangement is advantageous particularly in tagging
systems having a large and changing number of tags. The tag reader
12 thus periodically determines the identifiers of the RFID tags
14.sub.1 through 14.sub.n that are in the tagging system 10 and
uses this information in setting group addresses of the appropriate
size. The tag reader 12 can implement artificial intelligence in
determining optimum grouping based on the estimated number of
collisions in a reply message and the addresses present in those
messages may be used to determine group addresses.
[0068] After the group size in set, the tag reader 12 at a block
120 stores any data not involved in collisions in the memory 38 and
sets itself to the receive mode at a block 122. On the other hand,
if there were no collisions as determined at the block 114, the tag
reader 12 at the block 120 stores all data in the reply message,
and sets itself to the receive mode at the block 122.
[0069] As shown in FIG. 11, when the RFID tags 14.sub.1 through
14.sub.n receive a message (block 200), the RFID tags 14.sub.1
through 14.sub.n determine whether the message contains a group
address (202). If the RFID tags 14.sub.1 through 14.sub.n determine
that the message contains an individual address, the RFID tags
14.sub.1 through 14.sub.n compare the individual address to their
own identifiers on a data element-by-data element basis (block
204). A match requires that each data element in a tag's identifier
compare favorably with the corresponding data element in the
received individual address.
[0070] If a tag determines that all data elements of its identifier
compare favorably with all of the corresponding data elements in
the individual address of the received message (block 206), that
tag responds appropriately at a block 208. This response may
involve (i) transmitting back to the tag reader 12 the identifier
of the tag, and/or (ii) transmitting back to the tag reader 12 any
or all data stored in the memory 38, and/or (iii) taking such other
action as the received message requires.
[0071] If a tag determines that at least one of data elements of
its identifier does not compare favorably with a corresponding one
of the data elements contained in the individual address of the
received message (block 206), that tag ignores the received message
(block 210).
[0072] On the other hand, if the RFID tags 14.sub.1 through
14.sub.n determine that the received message contains a group
address (block 202), the RFID tags 14.sub.1 through 14.sub.n
compare the group address to their own identifiers on a data
element-by-data element basis (block 212). In making the comparison
at the block 212, however, a match is required only between the
non-x, or non-"don't care" data elements of the group address
contained in the received message and the corresponding data
elements in the identifiers of the RFID tags 14.sub.1 through
14.sub.n. Thus, a match requires that each data element in the
received individual address, except for the x data element
positions, compare favorably with a corresponding data element in a
tag's identifier.
[0073] For example, if the non-x, or non-"don't care" data elements
of the group address comprise a first set of data elements of the
group address, and if the x data elements of the group address
comprise a second set of data elements of the group address, then a
match requires a favorable comparison between only the first set of
data elements in the group address and the corresponding data
elements in the identifiers.
[0074] Those tags, which determine that all non-x, or non-"don't
care" data elements of the group address compare favorably with the
corresponding data elements in their identifiers, respond
appropriately at a block 216. As before, these responses may
involve (i) transmitting back to the tag reader 12 the identifiers
of the matching tags, and/or (ii) transmitting back to the tag
reader 12 any or all data stored in their memories 38, and/or (iii)
taking such other actions as the received message requires.
[0075] Those tags, which determine that at least one of the non-x,
or non-"don't care" data elements of the received group address
does not compare favorably with a corresponding one of the data
elements of their identifiers, ignore the received message (block
210).
[0076] In responding to the received message, the RFID tags
14.sub.1 through 14.sub.n may select a corresponding time slot on
any desired basis. For example, the responding tags may choose a
time slot on a pseudorandom basis, or they may choose a time slot
on the basis of all or a portion of their identifiers. In any
event, once the tags have either responded to or ignored the
received message, the tags wait for the next message.
[0077] Certain modifications of the present invention have been
disclosed above. Other modifications will occur to those practicing
in the art of the present invention. For example, although the RFID
tags 14.sub.1 through 14.sub.n are shown as microprocessor based
tags in FIG. 2, each of the RFID tags 14.sub.1 through 14.sub.n may
instead comprise one or more digital circuit elements, and/or one
or more programmable logic-arrays, and/or one or more dedicated
integrated circuits, etc.
[0078] Also, the tag reader 12 as described above has a range of
several hundred feet and could have an expected range of
approximately 500 feet. However, this range could be longer or
shorter depending on the application and/or other factors.
Similarly, the range given above for the second reader could be
other than as described above.
[0079] Additionally, the transmitter of the transceiver 30 of the
RFID tags 14.sub.1 through 14.sub.n is described above as the
frequency agile RF transmitter 32, and the receiver of the
transceiver 30 of the RFID tags 14.sub.1 through 14.sub.n is
described above as the direct sequence spread spectrum RF receiver
34. However, the RFID tags 14.sub.1 through 14.sub.n may instead
advantageously use other types of transmitters and receivers.
[0080] Moreover, the tagging system 10 may include only one tag
reader or more than one tag reader. Indeed the tagging system 10
may include any number of tag readers. Similarly, the RFID tags
14.sub.1 through 14.sub.n need only include one transceiver, or the
RFID tags 14.sub.1 through 14.sub.n may include more than one
transceiver. Indeed, each of the RFID tags 14.sub.1 through
14.sub.n may include any number of transceivers.
[0081] Furthermore, as described above, the tags that are addressed
respond to a received message. Alternatively, the addressed tags
may instead enter a quiescent or silent state such that it is the
non-addressed tags which respond to the received message.
[0082] Accordingly, the description of the present invention is to
be construed as illustrative only and is for the purpose of
teaching those skilled in the art the best mode of carrying out the
invention. The details may be varied substantially without
departing from the spirit of the invention, and the exclusive use
of all modifications which are within the scope of the appended
claims is reserved.
* * * * *